Reduced four-wave mixing optical fiber for wavelength-division multiplexing

ABSTRACT

An optical fiber includes a core for guiding light of a specified range of wavelengths therethrough, each wavelength in the specified range of wavelengths traveling through the core at a particular group velocity and the light potentially producing a nonlinear optical effect. The optical fiber also includes a cladding formed around the core for substantially containing the light within the core. The optical fiber further includes a predetermined amount of at least one dopant uniformly dispersed throughout the core such that no two distinct wavelengths in the specified range of wavelengths travel through the core at the same, particular group velocity, thereby causing the nonlinear optical effect to be suppressed.

RELATED APPLICATIONS

This application is a continuation application of copending U.S. patentapplication Ser. No. 10/808,916 filed Mar. 24, 2004, which is acontinuation of U.S. Pat. No. 6,738,548 filed Apr. 19, 2001, thedisclosure of which are both incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to optical fibers and, moreparticularly, to optical fibers in which the occurrence of four-wavemixing is reduced.

The advent of wavelength-division multiplexing (WDM) in opticalnetworking has increased the demands placed on optical fibers.Manufacturers are pressured to produce optical fibers that are capableof carrying optical signals on a wider range of wavelengths and overlonger distances with less loss than can be accomplished with presentlyavailable optical fibers. Concurrent developments in optical sources forWDM have increased the amount of optical power and bit rates that mustbe transmitted through the optical fiber.

The demand for higher optical power throughput and wider range oftransmitted wavelengths has led to an increase in the occurrence oferror due to nonlinear optical effects produced during light propagationthrough the optical fiber. Four-wave mixing (FWM) is especiallyproblematic in WDM networks because this nonlinear optical effect leadsto communication errors that cannot be easily removed by known solutionssuch as, for example, wavelength filtering and equalization.[1]

As is well-known in the art, FWM is the induced combination of threewavelengths to produce one or more new wavelengths. Two of the threecombining wavelengths can be degenerate such that FWM also includes thecombination of two wavelengths to produce one or more new wavelengths.Optical power is taken away from the combining wavelengths andtransferred to the new wavelengths in the FWM process. FWM is especiallyproblematic in optical communications if the new wavelengths produced bythe FWM process overlap the assigned wavelengths of existing WDMchannels because it is difficult to distinguish between the legitimateoptical data signals at these existing WDM channels and the error signalsuperimposed thereon as the result of FWM. Therefore, it is ofparticular concern to the practitioner of the art to suppress FWM.

It is also well-known that the efficiency of the FWM process increaseswhen the potentially combining wavelengths travel along the opticalfiber at the same group velocity over an extended distance. In otherwords, the longer the potentially combining wavelengths travel togetherdown the optical fiber at the same group velocity, the higher the riskof communication error resulting from FWM. A typical group indexprofile, generally indicated by reference number 10, as a function ofwavelength is shown in FIG. 1. As shown in FIG. 1, a conventionaloptical fiber typically exhibits a group index profile that is slightlyparabolic in shape over the optical fiber communication wavelengthrange. As indicated by dotted lines 12, 14 and 16, there exist pairs ofwavelengths, such as λ₁ and λ₂ shown in FIG. 1, that share the samegroup index values. In the example shown in FIG. 1, light signal ofwavelength λ₁ and light signal of wavelength λ₂ traveling through theoptical fiber will both “see” a group index value ofn₁=n_(g)(λ₁)=n_(g)(λ₂). Since the group velocity is related to the groupindex by the equation: v_(g)=c/n_(g), the group velocity of a lightsignal of wavelength λ₁ is equal to the group velocity of a light signalof wavelength λ₂. In this way, in conventional optical fibers, the groupvelocities of the shorter wavelengths in the wavelength range aregenerally the same as the group velocities of the longer wavelengths inthe wavelength range. As a result, the FWM efficiency for thecombination of the shorter wavelengths and the longer wavelengths in theoptical fiber communication wavelength range is high, therefore leadingto a high probability of potential error introduced in the transmittedoptical signals. In the example shown in FIG. 1, the FWM efficiency forthe combination of λ₁ with λ₂ is high, therefore leading to a highprobability of potential error occurring due to FWM.

Advances in chromatic dispersion shifting and reduction in opticalfibers have actually exacerbated the problem because FWM efficiencyincreases around the wavelength at which chromatic dispersion is zero.Increased optical power at the potentially combining wavelengths alsoincreases the FWM efficiency. Furthermore, increased variety ofwavelengths used at the WDM channels also exacerbate the FWM problembecause more optical wavelength combinations are available for the FWMprocess.

One known way to prevent the occurrence of FWM is to keep the opticalpower throughput low over the operating wavelength range of the WDMnetwork. However, reduction of optical power leads to problems such asthe cost associated with the need for additional repeaters to regeneratethe optical signals and the potential increase in bit error rates due tosignal weakness. The technological demand for increased distance betweenrepeaters, reduced cost and more reliable data transmission makes thisapproach impractical.

Another approach to FWM suppression is to provide small but non-zerochromatic dispersion over the operating wavelength range. By introducingvariation in the group velocities at different wavelengths in this way,FWM efficiency is reduced. This approach may be implemented usingspecialty optical fibers known in the art such as, for example, non-zerodispersion fiber (NZDF) and non-zero dispersion-shifted fiber (NZ-DSF).A dispersion compensation fiber (DCF) may also be used in this approach.Also known as a negative dispersion fiber, DCF is generally an opticalfiber whose chromatic dispersion decreases with increased wavelength(negative dispersion), as opposed to most other optical fibers whosechromatic dispersion increases at longer wavelengths (positivedispersion). By splicing predetermined lengths of DCF into an opticalnetwork that has been implemented using positive dispersion opticalfiber, the overall chromatic dispersion profile of the network can bemanipulated in such a way that a small but non-zero chromatic dispersionis present across the operating wavelength range thus suppressing FWM.

There exist commercially-available optical fibers which combine the twoaforedescribed approaches to FWM suppression. The LEAF fiber (which isan abbreviation for Large Effective Area Fiber), available from Corning,provides a larger effective mode area, in comparison to most availableNZ-DSF, through which the optical signals travel.[2] Thus, since theoptical power is spread over a larger area of the optical fiber than inconventional fibers, more optical power can be directed down the opticalfiber without inducing nonlinear effects such as FWM. In addition, theLEAF has the characteristics of NZ-DSF such that FWM efficiency isfurther reduced. The TrueWave XL fiber, manufactured by LucentTechnologies, also features a larger effective modal area as well asnegative dispersion properties.

A problem common to the aforedescribed prior art approaches is themanufacturing complexity of the specialty optical fibers. Careful designand precision fabrication are required in order to achieve the oftencomplex, radial refractive index profiles of these specialty fibers. Thetransmission characteristics of conventional optical fibers areestablished simply by the radius of the core and the relative values ofthe core refractive index and the cladding refractive index. However,the specialty optical fibers for FWM suppression require refractiveindex profiles that may vary linearly, parabolically, in steps or somecombination thereof in the radial direction from the center of the fibercore. Some specialty fibers even require multiple cladding layers ofdifferent materials. The design and fabrication of these specialtyfibers can be complicated and costly.[3]

The present invention provides an optical fiber which serves to resolvethe problems described above with regard to prior art optical fibers ina heretofore unseen and highly advantageous way and which provides stillfurther advantages.

SUMMARY OF THE INVENTION

As will be described in more detail hereinafter, there is disclosedherein an optical fiber including a core for guiding light of aspecified range of wavelengths therethrough, each wavelength in thespecified range of wavelengths traveling through the core at aparticular group velocity and the light potentially producing anonlinear optical effect. The optical fiber also includes a claddingformed around the core for substantially containing the light within thecore. The optical fiber further includes a predetermined amount of atleast one dopant uniformly dispersed throughout the core such that notwo distinct wavelengths in the specified range of wavelengths travelthrough the core at the same, particular group velocity, thereby causingthe nonlinear optical effect to be suppressed.

In another aspect of the invention, the core of the optical fiberincludes a range of group index values uniformly distributed throughoutthe core, each wavelength in the specified range of wavelengths beingassociated with a particular group index value in the range of groupindex values and the light potentially producing a nonlinear opticaleffect. The optical fiber also includes an amount of at least one dopantuniformly dispersed throughout the core such that no two distinctwavelengths in the specified range of wavelengths are associated withthe same, particular group index value in the range of group indexvalues, thereby causing the nonlinear optical effect to be suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be understood by reference to the followingdetailed description taken in conjunction with the drawings brieflydescribed below.

FIG. 1 is a graph illustrating the typical group index values as afunction of wavelength for a conventional optical fiber, shown here toillustrate the existence of degenerate group index values for twodifferent wavelengths.

FIG. 2A is a cross-sectional view of one embodiment of an optical fibermanufactured in accordance with the present invention shown here toillustrate the internal structure of the optical fiber.

FIG. 2B is a graph illustrating the distribution of refractive indexvalues across a diameter of the optical fiber shown in FIG. 2A.

FIG. 3 is a graph illustrating the refractive index values as a functionof photon energy for undoped silica.

FIG. 4A is a graph illustrating the refractive index values as afunction of photon energy for an optical fiber with a silica-based corethat has been doped uniformly throughout with 20 mol % of P₂O₅.

FIG. 4B is a graph illustrating the mode index values as a function ofphoton energy for an optical fiber with a silica-based core that hasbeen doped uniformly throughout with 20 mol % of P₂O₅.

FIG. 4C is a graph illustrating the group index values as a function ofphoton energy for an optical fiber with a silica-based core that hasbeen doped uniformly throughout with 20 mol % of P₂O₅.

FIG. 5A is a graph illustrating the refractive index values as afunction of photon energy for an optical fiber with a silica-based corethat has been doped uniformly throughout with 20 mol % of GeO₂ and asilica-based cladding that has been doped uniformly throughout with 20mol % of B₂O₃.

FIG. 5B is a graph illustrating the mode index values as a function ofphoton energy for an optical fiber with a silica-based core that hasbeen doped uniformly throughout with 20 mol % of GeO₂ and a silica-basedcladding that has been doped uniformly throughout with 20 mol % of B₂O₃.

FIG. 5C is a graph illustrating the group index values as a function ofphoton energy for an optical fiber with a silica-based core that hasbeen doped uniformly throughout with 20 mol % of GeO₂ and a silica-basedcladding that has been doped uniformly throughout with 20 mol % of B₂O₃.

DETAILED DESCRIPTION

Turning again to the drawings, attention is immediately directed to FIG.2A, which illustrates one embodiment of an optical fiber, generallyindicated by the reference numeral 20. Optical fiber 20 includes a core22 surrounded by a cladding 24. The dimensions of core 22 and cladding24 are similar to those of a conventional single mode fiber, whichcommonly has a core diameter of 4 μm and a cladding diameter of 125 μm.Core 22 and cladding 24 are based on silica glass (SiO₂) or a similarmaterial. Core 22 and cladding 24 are designed to have different valuesof refractive index such that light of a specified range of wavelengthsis guided through the optical fiber.

The relative values of the refractive index of air, cladding and coreare shown in FIG. 2B by line 26. As in the case of conventional singlemode fiber, the refractive index value of the cladding (n_(cladding)) isrelatively higher than that of air (n_(air)), and the core is composedof a material with a refractive index value (n_(core)) that is higherthan n_(cladding). It should be noted that the value of refractive indexof a given material is dependent on the properties of the material aswell as on the wavelength λ of light transmitted through that material.That is, each wavelength is associated with its own refractive indexvalue such that refractive index value is a function of λ. Hence, line26 of FIG. 2B represents the radial refractive index profile of opticalfiber 20 for a particular wavelength of light. The difference in thevalues of refractive index between core 22 and cladding 24 is designedsuch that light of a specified range of wavelengths is guided along thecore. For example, in WDM applications to date, it is desirable to beable to guide light of the wavelength range λ=1.29 μm to 1.60 μm throughthe optical fiber.

By noting that 1 electron volt (eV) of photon energy corresponds to theenergy of a photon of wavelength λ=1.24 μm, this wavelength range isequivalent to the photon energy range of E_(p)=0.775 eV to 0.961 eV. Inthis case, the refractive index value can be expressed as a function ofE_(p). A graph of the refractive index value versus photon energy forsilica over the photon energy range of E_(p)=0.775 eV to 0.961 eV isshown in FIG. 3. The graph shown in FIG. 3 has been calculated using atwo-term Sellmeier model as described in C. R. Hammond, “Silica-basedbinary glass systems: wavelength dispersive properties and compositionin optical fibres,” Optical and Quantum Electronics, vol. 10, 1978, pp.163-170 (Hammond), which is herein incorporated by reference. The unitsof photon energy will be used in the discussion which followshereinafter.

Continuing to refer to FIG. 3, curve 50, which corresponds to therefractive index value as a function of photon energy over the photonenergy range of interest, is generated according to the two-termSellmeier model as described in Hammond: $\begin{matrix}{{{n\left( E_{p} \right)}^{2} - 1} = {\frac{E_{d}E_{o}}{E_{o}^{2} - E_{p}^{2}} + \frac{E_{d}^{\prime}E_{o}^{\prime}}{E_{o}^{\prime 2} - E_{p}^{2}}}} & {{Eq}.\quad(1)}\end{matrix}$where E_(d) and E_(o) are the dispersion energy and the effectiveoscillator energy in the visible and near-infrared wavelengths,respectively, and E′_(d) and E′_(o) are respectively the dispersionenergy and the effective oscillator energy adjustment terms in theinfrared wavelengths. The values of E_(d), E_(o), E′_(d) and E′_(o) asgiven in Table I of Hammond are used in generating curve 50.

In general, the group index which corresponds to the group velocity atwhich light of a given photon energy will travel through a material, isalso dependent on the photon energy and properties of the material. Thegroup index can be calculated from the refractive index value n(E_(p))using the expression: $\begin{matrix}{{n_{group}\left( E_{p} \right)} = {{n\left( E_{p} \right)} + {E_{p} \cdot \frac{\mathbb{d}{n\left( E_{p} \right)}}{\mathbb{d}\left( E_{p} \right)}}}} & {{Eq}.\quad(2)}\end{matrix}$and the group velocity as a function of photon energy can then beexpressed as: $\begin{matrix}{{v_{group}\left( E_{p} \right)} = {\frac{c}{n_{group}\left( E_{p} \right)}.}} & {{Eq}.\quad(3)}\end{matrix}$

As previously observed, the occurrence of FWM increases when thepotentially combining wavelengths travel along the fiber at the samegroup velocity over an extended distance. Furthermore, it is also notedthat the group velocity is dependent on the group indexn_(group)(E_(p)), which is in turn dependent on the properties of thematerial through which the light is traveling. Therefore, by adjustingthe material properties of core 22 and/or cladding 24, it is possible toachieve a group index profile in which no two photon energies (orwavelengths) are associated with the same group index, and hence groupvelocity, over the photon energy range of interest. In this way, theoccurrence of FWM is reduced in optical fiber 20.

Returning to FIG. 2A, core 22 of optical fiber 20 includes at least onedopant (not shown) uniformly dispersed throughout the core. The dopantmaterial may be commonly used dopants used in fabricating conventionaloptical fiber including, but not limited to, phosphorus oxide (P₂O₅) andgermania (GeO₂). The amount of dopant included in the core is determinedso as to ensure that no two wavelengths in the specified range ofwavelengths are associated with the same refractive index value. Inaddition, a cladding dopant may be dispersed throughout cladding 24 soas to further reduce the FWM efficiency. A material suitable for use asthe cladding dopant includes, but is not limited to, boron oxide B₂O₃.

Attention is now directed to FIGS. 4A-4C illustrating the refractiveindex, mode index and group index values as a function of photon energyfor an optical fiber including a silica-based core doped with 20 mol %of P₂O₅ and a silica cladding according to the present invention. Thegraphs are generated using the abovementioned two-term Sellmeier modelas shown in Eq. 1 with a doping level dependency introduced into thedispersion and oscillator energy terms E_(d), E_(o), E′_(d) and E′_(o)so as to take into account the material composition of the core and thecladding. The dispersion and oscillator terms for the core, in thiscase, are dependent on a variable x, which corresponds to the molecularratio of silica to dopant (x SiO₂:1 P₂O₅, etc.). For example, x=4 for amaterial of a silica base doped with 20 mol % of P₂O₅. Then, thedispersion and oscillator energies are expressed as:E _(o)(x)=E _(o)(dopant)+U(x)[E _(o)(base)−E _(o)(dopant)]  Eq. (4a)andE _(d)(x)=E _(d)(dopant)+V(x)[E _(d)(base)−E _(d)(dopant)]  Eq. (4b)where U(x) and V(x) are the bond fraction and cation fraction,respectively. U(x) and V(x) are determined according to the compositionof the molecules with U(x)=V(x)=1 for SiO₂ and U(x)=V(x)=x/(x+2) withP₂O₅ as the dopant, according to Hammond. In this way, curve 100corresponding to the refractive index value as a function of photonenergy as shown in FIG. 4A is generated by combining Eq. (1) with Eqs.(4a) and (4b).

Turning now to FIG. 4B, curve 110 represents calculated mode indexvalues as a function of photon energy in the case where the opticalfiber is composed of a P₂O₅-doped silica core with a diameter of 4 μmand a silica cladding. Mode index n_(mode)(E_(p)) is a value whichcorresponds to the index of refraction as seen by an optical mode of aparticular photon energy propagating through the core an optical fiber.Mode index differs from the refractive index or group index because modeindex takes into account the finite diameter of the core as well as thedifference in the refractive index values between the core and thecladding of the optical fiber. For a system in which a material with aparticular refractive index value is surrounded by another material witha different refractive index value, the group index must be calculatedfrom the mode index. The mode index is expressed as:n _(mode)(E _(p))=n _(cladding)(E _(p))+β·[n _(core) (E _(p))−n_(cladding)(E _(p))]  Eq. (5)where β=[(1.1428−0.9960)/V ]²≈0.004074 to an accuracy of 0.2% using theapproximation that the V=ka (n_(core) ²−n_(cladding) ²)=2.3, wherek=wave number and a=core diameter=4 μm. In the present case of theP₂O₅-doped silica core optical fiber with silica cladding,n_(cladding)(E_(p)) is n(E_(p)) as shown in FIG. 3, and n_(core)(E_(p))is the refractive index as a function of photon energy as shown in FIG.4A. Mode index, as represented by curve 110 in FIG. 4B, becomes thebasis from which the group index is calculated.

Referring now to FIG. 4C, curve 120 represents the group index value asa function of photon energy for the P₂O₅-doped silica core optical fiberwith silica cladding. Curve 120 is generated using Eq. (2), replacingn(E_(p)) with n_(mode)(E_(p)). As can be seen in FIG. 4C, no two valuesof photon energy are associated with the same value of group index.Therefore, by doping the core of a silica-based optical fiber with 20mol % of P₂O₅, as in the case shown in FIG. 4C, it is ensured that notwo photon energies (or wavelengths) of light travel through the opticalfiber at the same group velocity and thus FWM is reduced.

The procedure used to generate FIGS. 4A-4C can be used to calculate theamount of dopant required to achieve the desired FWM-reduction effect.Different amounts of dopant, different dopant materials and combinationsof dopants can be inserted into Eqs. (1)-(5) to determine if anothermolecular concentration or a different dopant may be used to accomplishthe effect of tailoring the group index curve so that no two photonenergies of light travel at the same group velocity through the opticalfiber. In this way, combinations of core and cladding dopants and theirconcentrations for achieving non-degenerate group index values over thephoton energy range of interest are calculated.

It should be emphasized that the optical fiber with the refractive, modeand group index curves as shown in FIGS. 4A-4C has a simple, radialrefractive index profile as shown in FIG. 2B. The dimensions of thisoptical fiber fabricated according to the present invention are similarto those of conventional single mode optical fiber with a difference inthat the doping levels of the core has been tailored to yield a groupindex curve as shown in FIG. 4C. Therefore, the manufacturability of theoptical fiber fabricated according to the present invention is improvedwith respect to that of the more complex, prior art specialty opticalfibers such as NZ-DSF and DCF because uniform doping levels are usedthroughout the core and the cladding in the present invention instead ofvarying the doping level to achieve ramped or curved refractive indexprofiles. The optical fiber of the present invention can be fabricatedusing essentially the same manufacturing procedures as conventionaloptical fiber (with a difference in that a predetermined, uniform levelof one or more dopants is dispersed throughout the core) while reducingthe occurrence of FWM.

The refractive index, mode index and group index values as a function ofphoton energy of another optical fiber manufactured in accordance withthe present invention are illustrated in FIGS. 5A-5C. The optical fiberwith the index curves as shown in FIGS. 5A-5C includes a silica-basedcore doped with 20 mol % of GeO₂ and a silica-based cladding doped with20 mol % of B₂O₃. Although the specific refractive and mode index valuesshown in FIGS. 5A and 5B are different from those shown in FIGS. 4A and4B, respectively, the shapes of curves 200 and 210 are similar to curves100 and 110, respectively. In comparing FIG. 5C to FIG. 4C, however, itis noted that curve 220 of FIG. 5C, corresponding to the group indexvalues versus photon energy in the case of the GeO₂-doped core andB₂O₃-doped cladding combination, has a slightly more parabolic shape atthe higher photon energies than curve 120 shown in FIG. 4C. Therefore,the condition of having non-degenerate group index values over the rangeof photon energies is maintained only for photon energies less thanapproximately 0.925 eV, by inspection, for the GeO₂-doped core andB₂O₃-doped cladding combination. Hence, this particular combination ofdopants in the core and cladding will result in a fiber which iseffective in reducing FWM only over the range of approximately λ=1.34 to1.60 μm. However, the advantages of the simple, radial refractive indexprofile and hence the simplified manufacturing process of the opticalfiber as characterized in FIGS. 4A-4C are maintained in the opticalfiber whose index properties are illustrated in FIGS. 5A-5C.

Although only two specific examples of the present invention have beendescribed, since the optical fiber and associated method disclosedherein may be provided in a variety of different configurations and themethod may be practiced in a variety of different ways, it should beunderstood that the present invention may be embodied in many otherspecific ways without departing from the spirit or scope of theinvention. For example, the core and cladding of an optical fiber may bedoped in essentially unlimited number of ways using a variety ofdifferent dopants in a way which yields an optical fiber with a groupindex curve such that no two photon energies of light correspond to thesame group index (thus travel with the same group velocity) in theoptical fiber. Such modifications are considered to be within the scopeof the present invention so long as the teachings herein are applied.Therefore, the present examples are to be considered as illustrative andnot restrictive, and the invention is not to be limited to the detailsgiven herein, but may be modified within the scope of the appendedclaims.

REFERENCES

Refi, James J., “Optical Fibers for Optical Networking,” Bell LabsTechnical Journal, January-March 1999, pp. 246-261.

Able, Kevin M., “Optical-fiber designs evolve,” Lightwave Special Report(www.light-wave.com), February 1998.

Binh, Le Nguyen and Chung, Su-Vun, “Generalized approach to single-modedispersion-modified optical fiber design,” Optical Engineering, vol. 35,no. 8, August 1996, pp. 2250-2261.

Hammond, C. R., “Silica-based binary glass systems: wavelengthdispersive properties and composition in optical fibres,” Optical andQuantum Electronics, vol. 10, 1978, pp. 163-170.

1. An optical fiber comprising: a core for guiding light of a specifiedrange of wavelengths therethrough, said light traveling through saidoptical fiber with a first power and potentially producing a nonlinearoptical effect at said first power; a cladding formed around the corefore substantially containing the light within the core, said core andcladding being configured such that using only said core and saidcladding in said optical fiber provides a first probability of saidlight producing said nonlinear optical effect at said first power; and apredetermined amount of at least one dopant uniformly dispersedthroughout the core such that with said dopant, said optical fiberprovides a second probability not greater than said first probability ofsaid light producing said nonlinear optical effect at a given secondpower, wherein said second power is greater than said first power. 2.The optical fiber of claim 1 wherein said second power enables longerrange transmission of said range of wavelengths.
 3. The optical fiber ofclaim 1 wherein said second power enables a reduction in the bit errorrate.
 4. The optical fiber of claim 1 wherein said nonlinear opticaleffect is four-wave mixing.
 5. The optical fiber of claim 1 wherein saidcore is formed of SiO₂.
 6. The optical fiber of claim 5 wherein saiddopant is P₂O₅.
 7. The optical fiber of claim 5 wherein said dopant isGeO2.
 8. A method for reducing the nonlinear optical effects in anoptical fiber, said optical fiber including a core for guiding light ofa specified range of wavelengths therethrough, each wavelength in saidspecified range of wavelengths traveling through said optical fiber witha first power and potentially producing a nonlinear optical effect atsaid first power, said optical fiber further including a cladding formedaround the core for substantially containing the light within the core,said core and cladding being configured such that using only said coreand said cladding in said optical fiber provides a first probability ofsaid light producing said nonlinear optical effect at said first power,said method comprising: dispersing a predetermined amount of at leastone dopant uniformly dispersed throughout the core such that, with saiddopant, said optical fiber provides a second probability of said lightproducing said nonlinear optical effect at a second power, wherein saidsecond power is greater than said first power and said secondprobability is not greater than said first probability.
 9. The method ofclaim 5 wherein said second power enables longer range transmission ofsaid range of wavelengths.
 10. The method of claim 5 wherein said secondpower enables a reduction in the bit error rate.